Techniques for thermal cycling of DNA samples are known in the art. By performing a polymerase chain reaction (PCR), DNA can be amplified. It is desirable to cycle a specially constituted liquid biological reaction mixture through a specific duration and range of temperatures in order to successfully amplify the DNA in the liquid reaction mixture. Thermal cycling is the process of melting DNA, annealing short primers to the resulting single strands, and extending those primers to make new copies of double stranded DNA. The liquid reaction mixture is repeatedly put through this process of melting at high temperatures and annealing and extending at lower temperatures.
In a typical thermal cycling apparatus, a biological reaction mixture including DNA will be provided in a large number of sample wells on a thermal block assembly. It is desirable that the samples of DNA have temperatures throughout the thermal cycling process that are as uniform as reasonably possible. Even small variations in the temperature between one sample well and another sample well can cause a failure or undesirable outcome of the experiment. For instance, in quantitative PCR, one objective is to perform PCR amplification as precisely as possible by increasing the amount of DNA that generally doubles on every cycle; otherwise there can be an undesirable degree of disparity between the amount of resultant mixtures in the sample wells. If sufficiently uniform temperatures are not obtained by the sample wells, the desired doubling at each cycle may not occur. Although the theoretical doubling of DNA rarely occurs in practice, it is desired that the amplification occurs as efficiently as possible.
In addition, temperature errors can cause the reactions to improperly occur. For example, if the samples are not controlled to have the proper annealing temperatures, certain forms of DNA may not extend properly. This can result in the primers in the mixture annealing to the wrong DNA or not annealing at all. Moreover, by ensuring that all samples are uniformly heated, the dwell times at any temperature can be shortened, thereby speeding up the total PCR cycle time. By shortening this dwell time at certain temperatures, the lifetime and amplification efficiency of the enzyme are increased. Therefore, undesirable temperature errors and variations between the sample well temperatures should be decreased.
Prior art heating covers used in PCR heating equipment are simple, stiff, and relatively inexpensive. The prior art designs have mainly involved a stiff metal plate, a simple resistive heater, and an insulating cover. Because quantitative data was not generated, the heating covers did not have to control condensation in the biological samples as precisely as the heating covers used in QPCR equipment. Also, because optical data was not collected, the prior art heating cover designs were not complicated with the need to provide a means to excite and collect the optical data through the heating cover. Prior art heating covers used in QPCR heating equipment are mainly derived from their earlier PCR counterparts that provide a means for optical signal transmission, but, prior art heating covers are still mainly stiff designs which do not provide a uniform force distribution about the sample containers.
Prior art heating covers are difficult to use, expensive, complicated and do not provide uniform thermal contact or uniform force distribution about the sample wells. U.S. Pat. No. 5,475,610 discloses an instrument for performing PCR employing a cover which can be raised or lowered over a sample block. U.S. Pat. No. 5,475,610 does not disclose a cover assembly that is flexible to provide a more uniform thermal contact and force distribution on the sample tube caps. U.S. Pat. No. 5,928,907 discloses a system for carrying out real time fluorescence-based measurements of nucleic acid amplification products. U.S. Pat. No. 5,928,907 does not disclose a cover assembly that is flexible to provide a more uniform thermal contact and force distribution on the sample tube caps. The prior art does not disclose a cover assembly that is flexible to provide a more uniform thermal contact and force distribution on the sample tube caps.
In light of the foregoing, there is a need in the art for a flexible heating cover assembly that enhances the thermal response uniformity, efficiency, quality, reliability and controllability of the DNA sample wells in the thermal cycling apparatus.